PURPOSE: To enable accurate detection and control of water concentration independent of solvents, etc., of oils and fats, by controlling the water-content in an enzymatic reaction system of oils and fats using a water-concentration detection means including a sensor for water-concentration in liquid phase.

CONSTITUTION: Concentration of water in an enzymatic reaction system of oils and fats is detected with a water-concentration detection part 51 having a liquid-phase water-concentration sensor 51 (indicator for water-content in a solution), a solution temperature indicator 52, a water-content indicator for gas 53 and a gas temperature indicator 54. The indicator 51 for water-content in a solution is preferably a dew-point thermometer preferably having an Al2O3 thin film as a detector. The water-concentration in the solution can be determined by calculating the dew point in the solution at the temperature from the indications of the solution water-content indicator 51 and the solution temperature indicator 52.